Field of the Invention
The invention lies in the field of semiconductor technology and relates to a method for fabricating a trench isolation for electrically active components in a semiconductor component in which the trench isolation has at least one trench built with an oxide as an insulator. Such a method includes the following steps: a mask is applied to the semiconductor substrate; and a trench is subsequently formed in the semiconductor substrate by means of a dry etching step using at least one etching gas. During the dry etching process, polymers are produced which at least partly cover the side walls of the trench and thus, at least partially, protect the side walls against an etching attack from the etching gas.
A trench isolation for electrically active components, in particular, a shallow trench isolation produced in crystalline silicon, for example, has a trench filled with an oxide, for example. In the case of a dynamic semiconductor memory (DRAM), a capacitor element is arranged below the trench isolation. This capacitor element is delimited from the surrounding silicon by a collar that includes an oxide layer. A trench isolation of this type is disclosed for example in U.S. Pat. No. 5,389,559.
During the fabrication of this trench isolation, individual process steps are carried out at very high process temperatures at which the semiconductor substrate is exposed to a considerable thermal stress, in particular, during the heating and cooling phases. In this case, mechanical stresses arise in the semiconductor substrate, and can result in defects in the crystal lattice structure. At such sites, stress concentration sites arise which can trigger the formation of dislocations. Such dislocations can then lead to electrical defects, for example, leakage currents. These leakage currents cause a reduced retention time, with the result that the function of the active components (transistor, capacitor element, etc.) is restricted.
U.S. Pat. No. 5,807,789 discloses a method of the type mentioned in the introduction which can be used to create a trench isolation. The trench of the trench isolation has inclined side walls and round corners. In this method, the formation of polymers and their deposition on the side walls of the developing trench is utilized for protecting the side walls at least partly against an attack by the etching gases and thereby for achieving an etching profile with inclined side walls, and if appropriate, rounded corners. It is thus attempted to avoid sharp edges at the transition region between the bottom and the side walls of the trench since it has been found that such sharp edges can be the starting point for the above-described defects in the crystal lattice structure.
It is unfavorable that a mixture of chlorine-, bromine- and fluorine-containing gases is used in the method taught in U.S. Pat. No. 5,807,789. This mixture is very aggressive and is difficult to control. This means that the polymer formation cannot be influenced in the desired manner.
It is accordingly an object of the invention to provide a simple method for fabricating a trench isolation which overcomes the above-mentioned disadvantageous of the prior art methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating a trench isolation for electrically active components in a semiconductor component. A mask is applied to a semiconductor substrate. Subsequently, a trench having side walls is formed in the semiconductor substrate by performing a dry etching process using at least one etching gas such that during the dry etching process, polymers are produced that at least partly cover the side walls of the trench and thereby at least partially protect the side walls against an etching attack from the etching gas. The etching gas is provided with a compound that is selected from the group consisting of at least one hydrocarbon compound and a fluorinated hydrocarbon compound. The trench is filled with an insulating oxide.
The formulated object on which the invention is based is achieved by virtue of the fact that the etching gas contains at least one hydrocarbon compound and/or a fluorinated hydrocarbon compound.
These etching gases are easier to control, and in particular, can also be combined more easily with other etching steps which use related etching gases. Suitable etching gases are CH4, CHF3, CH2CF2 or CH3F, which may additionally contain NF3, SF6, N2, O2 and/or Ar.
Using the method to form a trench isolation enables reliable prevention of the formation of defects in the crystal lattice structure. Furthermore, the inclined (tapered) side walls have the particular advantage that the process of filling the STI with an oxide, which process is necessary after the dry etching process, is significantly improved.
In accordance with an added feature of the invention, the etching gas has NF3, CHF3 and N2 with a predominant proportion of CHF3 and little NF3.
In accordance with an additional feature of the invention, after the formation of the trench, a further dry etching step using a further etching gas is carried out, by means of which the corners of the transition between the bottom of the trench and the side walls thereof are rounded. The further dry etching step has an isotropic etching component. This produces inclined side walls with rounded corners. Preferably, during this dry etching step, the further etching gas contains NF3, CHF3, and N2 with a predominant proportion of NF3.
In accordance with another feature of the invention, the dry etching steps are assisted by magnetic-field-enhanced reactive ion etching with ICP (Inductive Coupled Plasma Source) or with ECR (Electrical Cyclotron Resonance).
In accordance with a further feature of the invention, a tapered mask is used for dry etching.
If the trench is formed only in one dry etching step, care should be taken to ensure that an oxide collar of a capacitor element is etched at the same rate as the remaining oxide or the surrounding semiconductor substrate. The oxide collar could possibly be situated at the bottom of the trench.
In accordance with a concomitant feature of the invention, instead of the hydrocarbon compounds and/or the fluorinated hydrocarbon compounds, it is also possible to use a gas mixture including HBr, Cl2, HCl and O2, N2 as additives.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in Method for fabricating a trench isolation for electrically active components, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.